1. Field of the Invention
The present invention relates to an image forming apparatus that is provided with a developing unit to develop, as a toner image, an electrostatic latent image formed on an image carrier, such as a photoreceptor.
2. Description of the Related Art
Recently, there have been prevailing various kinds of image forming apparatuses, such as a copier, a printer, a facsimile, and a multi-functional peripheral, that form an image on a transfer material, such as a sheet. Each of such image forming apparatuses exposes a charged photoreceptor based on image data so as to form an electrostatic latent image of the image data thereon; causes developer (toner) to adhere onto the electrostatic latent image on the photoreceptor so as to develop the electrostatic latent image as a toner image; and transfers the toner image on the photoreceptor onto a transfer material, such as a sheet, so as to form an image on the transfer material.
In such image forming apparatuses, a developing unit, having a role of developing an electrostatic latent image formed on a photoreceptor as a toner image, with developer (toner), agitates the developer so as to be charged by means of friction, and supplies the charged developer to a developing roller so as to be born thereon. The developing roller causes, by being rotated, the developer born thereon to adhere onto an electrostatic latent image portion of a photoreceptor.
In a case where a printing ratio of an image to be formed on a transfer material according to image data (ratio of the area of an image to be formed on a transfer material according to image data to the area of the transfer material on which the image is formed) is relatively low, the amount of developer to be consumed for the image formation is relatively low. For this reason, while a relatively large amount of sufficiently-charged developer exists in the developing unit, the amount of developer to be newly replenished into the developing unit is relatively small, so that the sufficiently-charged developer is supplied to the developing roller.
However, when the printing ratio rises, the consumption amount of developer increases, and it becomes necessary to replenish a relatively large amount of new developer into the developing unit. If a state that the printing ratio is relatively high continues, it occurs that newly replenished developer is supplied onto the developing roller in a state of having been insufficiently agitated, i.e., insufficiently charged. Consequently, a fogging phenomenon that developer adheres onto portions other than the electrostatic latent image on the photoreceptor occurs.
Conventionally, as a technique to suppress the fogging phenomenon, the technique to increase the rotation speed of an agitating member that agitates developer in a developing unit has been proposed (for instance, refer to Japanese Patent Laid-Open Publication No. 2011-209464).
However, according to the technique set forth in Japanese Patent Laid-Open Publication No. 2011-209464, since the action of the agitating member for agitating the developer so as to be charged and the action thereof for supplying the charged developer onto a developing roller are not independent from each other, when a large amount of new developer is replenished to the developing unit, such a case occurs that newly replenished developer is supplied onto the developing roller without having been sufficiently agitated by the agitating member. For this reason, when a large amount of new developer is replenished to the developing unit, it occurs that insufficiently charged developer is supplied onto the developing roller, and as a result, the fogging phenomenon cannot be suppressed.